Network Working Group P. Saint-Andre
Internet-Draft J. Miller
Expires: September 24, 2003 Jabber Software Foundation
March 26, 2003
XMPP Core
draft-ietf-xmpp-core-06
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the core features of the Extensible Messaging
and Presence Protocol (XMPP), a protocol for streaming XML in near-
real-time that is used mainly for the purpose of instant messaging
(IM) and presence by the servers, clients, and other applications
that comprise the Jabber network.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Discussion Venue . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Intellectual Property Notice . . . . . . . . . . . . . . . . 5
2. Generalized Architecture . . . . . . . . . . . . . . . . . . 6
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Server . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Client . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Gateway . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 Network . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Addressing Scheme . . . . . . . . . . . . . . . . . . . . . 8
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Domain Identifier . . . . . . . . . . . . . . . . . . . . . 8
3.3 Node Identifier . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Resource Identifier . . . . . . . . . . . . . . . . . . . . 9
4. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Restrictions . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3 Stream Attributes . . . . . . . . . . . . . . . . . . . . . 11
4.4 Namespace Declarations . . . . . . . . . . . . . . . . . . . 12
4.5 Stream Features . . . . . . . . . . . . . . . . . . . . . . 13
4.6 Stream Errors . . . . . . . . . . . . . . . . . . . . . . . 14
4.6.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.6.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.6.3 Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6.4 Extensibility . . . . . . . . . . . . . . . . . . . . . . . 16
4.7 Simple Streams Example . . . . . . . . . . . . . . . . . . . 16
5. Stream Encryption . . . . . . . . . . . . . . . . . . . . . 18
5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3 Client-to-Server Example . . . . . . . . . . . . . . . . . . 19
5.4 Server-to-Server Example . . . . . . . . . . . . . . . . . . 21
6. Stream Authentication . . . . . . . . . . . . . . . . . . . 24
6.1 SASL Authentication . . . . . . . . . . . . . . . . . . . . 24
6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1.2 Narrative . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.1.3 SASL Definition . . . . . . . . . . . . . . . . . . . . . . 26
6.1.4 Client-to-Server Example . . . . . . . . . . . . . . . . . . 27
6.1.5 Server-to-Server Example . . . . . . . . . . . . . . . . . . 29
6.2 Dialback Authentication . . . . . . . . . . . . . . . . . . 32
6.2.1 Dialback Protocol . . . . . . . . . . . . . . . . . . . . . 34
7. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . 37
7.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 37
7.2 Common Attributes . . . . . . . . . . . . . . . . . . . . . 37
7.2.1 to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
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7.2.2 from . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
7.2.3 id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
7.2.4 type . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
7.2.5 xml:lang . . . . . . . . . . . . . . . . . . . . . . . . . . 38
7.3 Message Stanzas . . . . . . . . . . . . . . . . . . . . . . 38
7.3.1 Types of Message . . . . . . . . . . . . . . . . . . . . . . 39
7.3.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.4 Presence Stanzas . . . . . . . . . . . . . . . . . . . . . . 40
7.4.1 Types of Presence . . . . . . . . . . . . . . . . . . . . . 41
7.4.2 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 41
7.5 IQ Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 43
7.5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 43
7.5.2 Types of IQ . . . . . . . . . . . . . . . . . . . . . . . . 43
7.5.3 Children . . . . . . . . . . . . . . . . . . . . . . . . . . 44
7.6 Extended Namespaces . . . . . . . . . . . . . . . . . . . . 44
7.7 Stanza Errors . . . . . . . . . . . . . . . . . . . . . . . 45
7.7.1 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7.7.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7.7.3 Conditions . . . . . . . . . . . . . . . . . . . . . . . . . 46
7.7.4 Extensibility . . . . . . . . . . . . . . . . . . . . . . . 47
8. XML Usage within XMPP . . . . . . . . . . . . . . . . . . . 48
8.1 Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . 48
8.2 Validation . . . . . . . . . . . . . . . . . . . . . . . . . 48
8.3 Character Encodings . . . . . . . . . . . . . . . . . . . . 48
8.4 Inclusion of Text Declaration . . . . . . . . . . . . . . . 48
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 49
10. Internationalization Considerations . . . . . . . . . . . . 50
11. Security Considerations . . . . . . . . . . . . . . . . . . 51
11.1 High Security . . . . . . . . . . . . . . . . . . . . . . . 51
11.2 Client-to-Server Communications . . . . . . . . . . . . . . 51
11.3 Server-to-Server Communications . . . . . . . . . . . . . . 51
11.4 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . 52
11.5 Mandatory to Implement Technologies . . . . . . . . . . . . 52
References . . . . . . . . . . . . . . . . . . . . . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 55
A. XML Schemas . . . . . . . . . . . . . . . . . . . . . . . . 56
A.1 Streams namespace . . . . . . . . . . . . . . . . . . . . . 56
A.2 TLS namespace . . . . . . . . . . . . . . . . . . . . . . . 57
A.3 SASL namespace . . . . . . . . . . . . . . . . . . . . . . . 57
A.4 Dialback namespace . . . . . . . . . . . . . . . . . . . . . 58
A.5 Client namespace . . . . . . . . . . . . . . . . . . . . . . 59
A.6 Server namespace . . . . . . . . . . . . . . . . . . . . . . 62
A.7 Stream error namespace . . . . . . . . . . . . . . . . . . . 65
A.8 Stanza error namespace . . . . . . . . . . . . . . . . . . . 66
B. Provisional Namespace Names . . . . . . . . . . . . . . . . 68
C. Revision History . . . . . . . . . . . . . . . . . . . . . . 69
C.1 Changes from draft-ietf-xmpp-core-05 . . . . . . . . . . . . 69
C.2 Changes from draft-ietf-xmpp-core-04 . . . . . . . . . . . . 69
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C.3 Changes from draft-ietf-xmpp-core-03 . . . . . . . . . . . . 69
C.4 Changes from draft-ietf-xmpp-core-02 . . . . . . . . . . . . 69
C.5 Changes from draft-ietf-xmpp-core-01 . . . . . . . . . . . . 70
C.6 Changes from draft-ietf-xmpp-core-00 . . . . . . . . . . . . 70
C.7 Changes from draft-miller-xmpp-core-02 . . . . . . . . . . . 70
Full Copyright Statement . . . . . . . . . . . . . . . . . . 72
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1. Introduction
1.1 Overview
The Extensible Messaging and Presence Protocol (XMPP) is an open XML
[1] protocol for near-real-time messaging, presence, and request-
response services. The protocol was developed originally within the
Jabber community starting in 1998, and has continued to evolve under
the auspices of the Jabber Software Foundation [2] (since June 2001)
and the XMPP WG (since November 2002). The current document defines
the core features of XMPP; XMPP IM [3] defines the extensions
necessary to provide the instant messaging (IM) and presence
functionality defined in RFC 2779 [4].
1.2 Terminology
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [5].
1.3 Discussion Venue
The authors welcome discussion and comments related to the topics
presented in this document. The preferred forum is the
mailing list, for which archives and subscription
information are available at .
1.4 Intellectual Property Notice
This document is in full compliance with all provisions of Section 10
of RFC 2026. Parts of this specification use the term "jabber" for
identifying namespaces and other protocol syntax. Jabber[tm] is a
registered trademark of Jabber, Inc. Jabber, Inc. grants permission
to the IETF for use of the Jabber trademark in association with this
specification and its successors, if any.
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2. Generalized Architecture
2.1 Overview
Although XMPP is not wedded to any specific network architecture, to
this point it has usually been implemented via a typical client-
server architecture, wherein a client utilizing XMPP accesses a
server over a TCP [6] socket.
The following diagram provides a high-level overview of this
architecture (where "-" represents communications that use XMPP and
"=" represents communications that use any other protocol).
C1 - S1 - S2 - C3
/ \
C2 - G1 = FN1 = FC1
The symbols are as follows:
o C1, C2, C3 -- XMPP clients
o S1, S2 -- XMPP servers
o G1 -- A gateway that translates between XMPP and the protocol(s)
used on a foreign (non-XMPP) messaging network
o FN1 -- A foreign messaging network
o FC1 -- A client on a foreign messaging network
2.2 Server
A server acts as an intelligent abstraction layer for XMPP
communications. Its primary responsibilities are to manage
connections from or sessions for other entities (in the form of XML
streams to and from authorized clients, servers, and other entities)
and to route appropriately-addressed XML data "stanzas" among such
entities over XML streams. Most XMPP-compliant servers also assume
responsibility for the storage of data that is used by clients (e.g.,
contact lists for users of XMPP-based IM applications); in this case,
the XML data is processed directly by the server itself on behalf of
the client and is not routed to another entity. Compliant server
implementations MUST ensure in-order processing of XML stanzas
received from connected clients, servers, and services.
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2.3 Client
Most clients connect directly to a server over a TCP socket and use
XMPP to take full advantage of the functionality provided by a server
and any associated services, although it must be noted that there is
no necessary coupling of an XML stream to a TCP socket (e.g., a
client COULD connect via HTTP polling or some other mechanism).
Multiple resources (e.g., devices or locations) MAY connect
simultaneously to a server on behalf of each authorized client, with
each resource connecting over a discrete TCP socket and
differentiated by the resource identifier of a JID (Section 3) (e.g.,
user@domain/home vs. user@domain/work). The port registered with
the IANA [7] for connections between a Jabber client and a Jabber
server is 5222. For further details about client-to-server
communications expressly for the purpose of instant messaging and
presence, refer to XMPP IM [3].
2.4 Gateway
A gateway is a special-purpose server-side service whose primary
function is to translate XMPP into the protocol(s) of another
messaging system, as well as to translate the return data back into
XMPP. Examples are gateways to SIMPLE, Internet Relay Chat (IRC),
Short Message Service (SMS), SMTP, and foreign instant messaging
networks such as Yahoo!, MSN, ICQ, and AIM. Communications between
gateways and servers, and between gateways and the foreign messaging
system, are not defined in this document.
2.5 Network
Because each server is identified by a network address (typically a
DNS hostname) and because server-to-server communications are a
straightforward extension of the client-to-server protocol, in
practice the system consists of a network of servers that inter-
communicate. Thus user-a@domain1 is able to exchange messages,
presence, and other information with user-b@domain2. This pattern is
familiar from messaging protocols (such as SMTP) that make use of
network addressing standards. Upon opening a TCP socket on the IANA-
registered port 5269, there are two methods for negotiating a
connection between any two servers: server dialback (Section 6.2) and
SASL authentication (Section 6.1).
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3. Addressing Scheme
3.1 Overview
An entity is anything that can be considered a network endpoint
(i.e., an ID on the network) and that can communicate using XMPP.
All such entities are uniquely addressable in a form that is
consistent with RFC 2396 [8]. In particular, a valid Jabber
Identifier (JID) contains a set of ordered elements formed of a
domain identifier, node identifier, and resource identifier in the
following format: [node@]domain[/resource].
All JIDs are based on the foregoing structure. The most common use
of this structure is to identify an IM user, the server to which the
user connects, and the user's active session or connection (e.g., a
specific client) in the form of user@domain/resource. However, node
types other than clients are possible; for example, a specific chat
room offered by a multi-user chat service could be addressed as
(where "room" is the name of the chat room and
"service" is the hostname of the multi-user chat service) and a
specific occupant of such a room could be addressed as (where "nick" is the occupant's room nickname). Many other JID
types are possible (e.g., could be a server-side
script or service).
3.2 Domain Identifier
The domain identifier is the primary identifier and is the only
REQUIRED element of a JID (a mere domain identifier is a valid JID).
It usually represents the network gateway or "primary" server to
which other entities connect for XML routing and data management
capabilities. However, the entity referenced by a domain identifier
is not always a server, and may be a service that is addressed as a
subdomain of a server and that provides functionality above and
beyond the capabilities of a server (a multi-user chat service, a
user directory, a gateway to a foreign messaging system, etc.).
The domain identifier for every server or service that will
communicate over a network SHOULD resolve to a Fully Qualified Domain
Name. A domain identifier MUST conform to RFC 952 [9] and RFC 1123
[10]. A domain identifier MUST be no more than 1023 bytes in length
and MUST conform to the nameprep [11] profile of stringprep [12].
3.3 Node Identifier
The node identifier is an optional secondary identifier. It usually
represents the entity requesting and using network access provided by
the server or gateway (i.e., a client), although it can also
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represent other kinds of entities (e.g., a multi-user chat room
associated with a multi-user chat service). The entity represented
by a node identifier is addressed within the context of a specific
domain; within IM applications of XMPP this address is called a "bare
JID" and is of the form .
A node identifier MUST be no more than 1023 bytes in length and MUST
conform to the nodeprep [13] profile of stringprep [12].
3.4 Resource Identifier
The resource identifer is an optional tertiary identifier. It
usually represents a specific session, connection (e.g., a device or
location), or object (e.g., a participant in a multi-user chat room)
belonging to the entity associated with a node identifier. An entity
may maintain multiple resources simultaneously.
A resource identifier MUST be no more than 1023 bytes in length and
MUST conform to the resourceprep [14] profile of stringprep [12].
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4. XML Streams
4.1 Overview
Two fundamental concepts make possible the rapid, asynchronous
exchange of relatively small payloads of structured information
between presence-aware entities: XML streams and XML stanzas:
Definition of XML stream: An XML stream is a container for the
exchange of XML elements between any two entities over a network.
An XML stream is negotiated from an initiating entity (usually a
client or server) to a receiving entity (usually a server),
normally over a TCP socket. An XML stream corresponds to the
initiating entity's session with the receiving entity. The start
of the XML stream is denoted unambiguously by an opening XML
tag with appropriate attributes and namespace
declarations. The end of the XML stream is denoted unambiguously
be a closing XML tag.
Definition of XML stanza: An XML stanza is a discrete semantic unit
of structured information that is sent from one entity to another
over an XML stream. An XML stanza exists at the direct child
level of the root element and is said to be well-
balanced if it matches production [43] content of the XML
specification [1]). The start of any XML stanza is denoted
unambiguously by the element start tag at depth=1 (e.g.,
), and the end of any XML stanza is denoted
unambiguously by the corresponding close tag at depth=1 (e.g.,
presence>). An XML stanza MAY contain child elements or CDATA
sections as necessary in order to convey the desired information.
Consider the example of a client's session with a server. In order
to connect to a server, a client must initiate an XML stream by
sending an opening tag to the server, optionally preceded by
a text declaration specifying the XML version supported and the
character encoding. The server SHOULD then reply with a second XML
stream back to the client, again optionally preceded by a text
declaration. Once the client has authenticated with the server (see
Section 6), the client MAY send an unlimited number of XML stanzas
over the stream to any recipient on the network. When the client
desired to close the stream, it simply sends a closing tag
to the server (alternatively, the session may be closed by the
server).
Thus a client's session with a server can be seen as two open-ended
XML documents that are built up through the accumulation of the XML
stanzas sent over the two XML streams (i.e., one from the client to
the server and one from the server to the client), and the root
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element can be considered the document entity for each
document. In essence, then, an XML stream acts as an envelope for
all the XML stanzas sent during a session. We can represent this
graphically as follows:
|-------------------|
| |
|-------------------|
| |
| |
| |
|-------------------|
| |
| |
| |
|-------------------|
| |
| |
| |
|-------------------|
| ... |
|-------------------|
| |
|-------------------|
4.2 Restrictions
XML streams are used to transport a subset of XML. Specifically, XML
streams SHOULD NOT contain processing instructions, predefined
entities (as defined in Section 4.6 of the XML specification [1]),
comments, or DTDs. Any such XML data SHOULD be ignored by a
compliant implementation.
4.3 Stream Attributes
The attributes of the stream element are as follows:
o to -- The 'to' attribute SHOULD be used only in the XML stream
from the initiating entity to the receiving entity, and MUST be
set to the XMPP address of the receiving entity. There SHOULD be
no 'to' attribute set in the XML stream by which the receiving
entity replies to the initiating entity; however, if a 'to'
attribute is included, it SHOULD be ignored by the initiating
entity.
o from -- The 'from' attribute SHOULD be used only in the XML stream
from the receiving entity to the initiating entity, and MUST be
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set to the XMPP address of the receiving entity granting access to
the initiating entity. There SHOULD be no 'from' attribute on the
XML stream sent from the initiating entity to the receiving
entity; however, if a 'from' attribute is included, it SHOULD be
ignored by the receiving entity.
o id -- The 'id' attribute SHOULD be used only in the XML stream
from the receiving entity to the initiating entity. This
attribute is a unique identifier created by the receiving entity
to function as a session key for the initiating entity's session
with the receiving entity. There SHOULD be no 'id' attribute on
the XML stream sent from the initiating entity to the receiving
entity; however, if an 'id' attribute is included, it SHOULD be
ignored by the receiving entity.
o version -- The 'version' attribute MAY be used in the XML stream
from the initiating entity to the receiving entity in order signal
compliance with the protocol defined herein; this is done by
setting the value of the attribute to "1.0". If the initiating
entity includes the version attribute and the receiving entity
supports XMPP 1.0, the receiving entity MUST reciprocate by
including the attribute in its response.
We can summarize these values as follows:
| initiating to receiving | receiving to initiating
------------------------------------------------------------
to | JID of receiver | ignored
from | ignored | JID of receiver
id | ignored | session key
version | signals XMPP 1.0 support | signals XMPP 1.0 support
4.4 Namespace Declarations
The stream element MAY contain namespace declarations as defined in
the XML namespaces specification [15].
A default namespace declaration ('xmlns') is REQUIRED and is used in
both XML streams in order to scope the allowable first-level children
of the root stream element for both streams. This namespace
declaration MUST be the same for the initiating stream and the
responding stream so that both streams are scoped consistently. The
default namespace declaration applies to the stream and all stanzas
sent within a stream.
A stream namespace declaration (e.g., 'xmlns:stream') is REQUIRED in
both XML streams. A compliant entity SHOULD accept any namespace
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prefix on the element; however, for historical reasons some
entities MAY accept only a 'stream' prefix, resulting in the use of a
element as the stream root. The name of the stream
namespace MUST be "http://etherx.jabber.org/streams".
Since XML streams function as containers for any XML stanzas sent
asynchronously between network endpoints, it should be possible to
scope an XML stream with any default namespace declaration (i.e., it
should be possible to send any properly-namespaced XML stanza over an
XML stream). At a minimum, a compliant implementation MUST support
the following two namespaces (for historical reasons, existing
implementations MAY support only these two default namespaces):
o jabber:client -- this default namespace is declared when the
stream is used for communications between a client and a server
o jabber:server -- this default namespace is declared when the
stream is used for communications between two servers
The jabber:client and jabber:server namespaces are nearly identical
but are used in different contexts (client-to-server communications
for jabber:client and server-to-server communications for
jabber:server). The only difference between the two is that the 'to'
and 'from' attributes are OPTIONAL on stanzas sent within
jabber:client, whereas they are REQUIRED on stanzas sent within
jabber:server. If a compliant implementation accepts a stream that
is scoped by the 'jabber:client' or 'jabber:server' namespace, it
MUST support all three core stanza types (message, presence, and IQ)
as described herein and defined in the schema.
4.5 Stream Features
The root stream element MAY contain a features child element (e.g.,
if the stream namespace prefix is 'stream'). This
is used to communicate generic stream-level capabilities including
stream-level features that can be negotiated as the streams are set
up. If the initiating entity sends a "version='1.0'" flag in its
initiating stream element, the receiving entity MUST send a features
child element to the initiating entity if there are any capabilities
that need to be advertised or features that can be negotiated for the
stream. Currently this is used for SASL and TLS negotiation only,
but it could be used for other negotiable features in the future
(usage is defined under Stream Encryption (Section 5) and Stream
Authentication (Section 6) below). If an entity does not understand
or support some features, it SHOULD ignore them.
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4.6 Stream Errors
The root stream element MAY contain an error child element (e.g.,
if the stream namespace prefix is 'stream'). The
error child MUST be sent by a Jabber entity (usually a server rather
than a client) if it perceives that a stream-level error has
occurred.
4.6.1 Rules
The following rules apply to stream-level errors:
o It is assumed that all stream-level errors are unrecoverable;
therefore, if an error occurs at the level of the stream, the
entity that detects the error MUST send a stream error to the
other entity and then send a closing tag.
o If the error occurs while the stream is being set up, the
receiving entity MUST still send the opening and closing stream
tags and include the error element as a child of the stream
element. In this case, if the initiating entity provides an
unknown host in the 'to' attribute (or provides no 'to' attribute
at all), the server SHOULD provide the server's authoritative
hostname in the 'from' attribute of the stream header.
4.6.2 Syntax
The syntax for stream errors is as follows:
The value of the 'class' attribute must be one of the following:
o address -- the condition relates to the JID or domain to which the
stream was addressed
o format -- the condition relates to XML format or structure
o redirect -- the condition relates to a host redirection
o server -- the condition relates to the internal state of the
server
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The element MUST contain a child element that
specifies a particular stream-level error condition, as defined in
the next section. (Note: the XML namespace name
'urn:ietf:rfc:xmppcore-rfc-number:streams' that scopes the element adheres to the format defined in RFC 2648 [16].)
4.6.3 Conditions
The following stream-level error conditions are defined:
o -- the value of the 'to' attribute provided by the
initiating entity in the stream header corresponds to a hostname
that is no longer hosted by the server; the associated class is
"address".
o -- the value of the 'to' attribute provided by the
initiating entity in the stream header does not correspond to a
hostname that is hosted by the server; the associated class is
"address".
o -- the server has experienced a
misconfiguration or an otherwise-undefined internal server error
that prevents it from servicing the stream; the associated class
is "server".
o -- the stream namespace name is something
other than "http://etherx.jabber.org/streams"; the associated
class is "format".
o -- the server is resource-contrained and is
unable to service the stream; the associated class is "server".
o -- the server will not provide service to the
initiating entity but is redirecting traffic to another host; this
element SHOULD contain CDATA specifying the alternate hostname or
IP address to which the initiating entity MAY attempt to connect;
the associated class is "redirect".
o -- the server is being shut down and all active
streams are being closed; the associated class is "server".
o -- the initiating entity has sent a
first-level child of the stream that is not supported by the
server; the associated class is "format".
o -- the value of the 'version' attribute
provided by the initiating entity in the stream header specifies a
version of XMPP that is not supported by the server; this element
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MAY contain CDATA specifying the XMPP version(s) supported by the
server; the associated class is "format".
o -- the initiating entity has sent XML that
is not well-formed as defined by the XML specification [1]; the
associated class is "format".
4.6.4 Extensibility
If desired, an XMPP application MAY provide custom error information;
this MUST be contained in a properly-namespaced child of the element (i.e., the namespace name MUST NOT be one of the
namespace names defined herein).
4.7 Simple Streams Example
The following is a stream-based session of a client on a server
(where the "C" lines are sent from the client to the server, and the
"S" lines are sent from the server to the client):
A basic session:
C:
S:
... authentication ...
C:
C: Watson come here, I want you!
C:
S:
S: I'm on my way!
S:
C:
S:
These are in actuality a sending stream and a receiving stream, which
can be viewed a-chronologically as two XML documents:
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C:
C:
C: Watson come here, I want you!
C:
C:
S:
S:
S: I'm on my way!
S:
S:
A session gone bad:
C:
S:
C: Bad XML, no closing body tag!
S:
S:
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5. Stream Encryption
5.1 Overview
XMPP includes a method for securing the stream from tampering and
eavesdropping. This channel encryption method makes use of the
Transport Layer Security (TLS) [17] protocol, along with a "STARTTLS"
extension that is modelled on similar extensions for the IMAP [18],
POP3 [19], and ACAP [20] protocols as described in RFC 2595 [21].
The namespace identifier for the STARTTLS extension is 'http://
www.ietf.org/rfc/rfc2595.txt'. TLS may be used between any
initiating entity and any receiving entity (e.g., a stream from a
client to a server or from one server to another).
The following rules MUST be observed:
1. If the initiating entity is capable of using the STARTTLS
extension, it MUST include the "version='1.0'" flag in the
initiating stream header.
2. If the receiving entity is capable of using the STARTTLS
extension, it MUST send the element in the defined
namespace along with the list of features that it sends in
response to the opening stream tag received from the initiating
entity.
3. If the initiating entity chooses to use TLS for stream
encryption, TLS negotiation MUST be completed before proceeding
to authenticate the stream using SASL.
4. If the TLS negotiation is successful, TLS takes effect
immediately following the closing ">" character of the element for the client and immediately following the closing
">" character of the element for the server. A new
stream MUST then be initiated by the initiating entity.
5. If the TLS negotiation is successful, the receiving entity MUST
discard any knowledge obtained from the initiating entity before
TLS takes effect.
6. If the TLS negotiation is successful, the initiating entity MUST
discard any knowledge obtained from the receiving entity before
TLS takes effect.
7. If the TLS negotiation is successful, the receiving entity MUST
NOT offer the STARTTLS extension to the initiating entity along
with the other stream features that are offered when the stream
is restarted.
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8. Whether the TLS negotiation results in success or failure, the
initiating entity SHOULD continue with SASL negotiation.
9. If TLS is used for stream encryption, SASL MUST NOT be used for
anything but stream authentication (i.e., a security layer MUST
NOT be negotiated using SASL). Conversely, if a security layer
is to be negotiated via SASL, TLS MUST NOT be used.
5.2 Narrative
When an initiating entity secures a stream with a receiving entity,
the steps involved are as follows:
1. Then initiating entity opens a TCP connection and initiates the
stream by sending the opening XML stream header to the receiving
entity, including the "version='1.0'" flag.
2. The receiving entity responds by opening a TCP connection and
sending an XML stream header to then initiating entity.
3. The receiving entity offers the STARTTLS extension to then
initiating entity by sending it along with the list of supported
stream features.
4. Then initiating entity issues the STARTTLS command to instruct
the receiving entity that it wishes to begin a TLS negotiation to
secure the stream.
5. The receiving entity MUST reply with either an empty
element or an empty element, but keep the underlying
TCP connection open.
6. Then initiating entity begins a TLS negotiation in accordance
with RFC 2246 [17]. Upon completion of the negotiation, then
initiating entity initiates a new stream by sending a new opening
XML stream header to the receiving entity.
7. The receiving entity responds by sending an XML stream header to
then initiating entity along with the remaining available
features (but NOT including the STARTTLS element).
5.3 Client-to-Server Example
The following example shows the data flow for a client securing a
stream using STARTTLS.
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Step 1: Client initiates stream to server:
Step 2: Server responds by sending a stream tag to the client:
Step 3: Server sends the STARTTLS extension to the client along with
authentication mechanisms and any other stream features:
DIGEST-MD5PLAIN
Step 4: Client sends the STARTTLS command to the server:
Step 5: Server informs client to proceed:
Step 5 (alt): Server informs client that TLS negotiation has failed
(client SHOULD continue with stream authentication (Section 6)):
Step 6: Client and server complete TLS negotiation via TCP.
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Step 7: Client initiates a new stream to the server:
Step 8: Server responds by sending a stream header to the client
along with any remaining negotiatiable stream features:
DIGEST-MD5PLAINEXTERNAL
Step 9: Client SHOULD continue with stream authentication (Section
6).
5.4 Server-to-Server Example
By bilateral agreement, server administrators SHOULD choose to use
TLS between two domains for the purpose of securing server-to-server
communications.
The following example shows the data flow for two servers securing a
stream using STARTTLS.
Step 1: Server1 initiates stream to Server2:
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Step 2: Server2 responds by sending a stream tag to Server1:
Step 3: Server2 sends the STARTTLS extension to Server1 along with
authentication mechanisms and any other stream features:
DIGEST-MD5KERBEROS_V4
Step 4: Server1 sends the STARTTLS command to Server2:
Step 5: Server2 informs Server1 to proceed:
Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed
(Server1 SHOULD continue with stream authentication (Section 6)):
Step 6: Server1 and Server2 complete TLS negotiation via TCP.
Step 7: Server1 initiates a new stream to Server2:
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Step 8: Server2 responds by sending a stream header to Server1 along
with any remaining negotiatiable stream features:
DIGEST-MD5KERBEROS_V4EXTERNAL
Step 9: Server1 SHOULD continue with stream authentication (Section
6).
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6. Stream Authentication
XMPP includes two methods for enforcing authentication at the level
of XML streams. When one entity is already known to another (i.e.,
there is an existing trust relationship between the entities such as
that established when a user registers with a server or an
administrator configures a server to trust another server), the
preferred method for authenticating streams between the two entities
uses an XMPP adaptation of the Simple Authentication and Security
Layer (SASL) [22]. When there is no existing trust relationship
between two servers, some level of trust MAY be established based on
existing trust in DNS; the authentication method used in this case is
the server dialback protocol that is native to XMPP (no such ad-hoc
method is defined between a client and a server). If SASL is used
for server-to-server authentication, the servers MUST NOT use
dialback. Both SASL authentication and dialback are described in
this section.
Stream authentication is REQUIRED for all direct communications
between two entities; if an entity sends a stanza to an
unauthenticated stream, the receiving entity SHOULD silently drop the
stanza and MUST NOT process it.
6.1 SASL Authentication
6.1.1 Overview
The Simple Authentication and Security Layer (SASL) provides a
generalized method for adding authentication support to connection-
based protocols. XMPP uses a generic XML namespace profile for SASL
that conforms to section 4 ("Profiling Requirements") of RFC 2222
[22] (the namespace identifier for this protocol is 'http://
www.iana.org/assignments/sasl-mechanisms').
The following rules MUST be observed:
1. If TLS is used for stream encryption, SASL MUST NOT be used for
anything but stream authentication (i.e., a security layer MUST
NOT be negotiated using SASL). Conversely, if a security layer
is to be negotiated via SASL, TLS MUST NOT be used.
2. If the initiating entity is capable of authenticating via SASL,
it it MUST include the "version='1.0'" flag in the initiating
stream header.
3. If the receiving entity is capable of accepting authentications
via SASL, it MUST send one or more authentication mechanisms
within a element in response to the opening stream
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tag received from the initiating entity.
4. If the SASL negotiation involves negotiation of a security layer,
the receiving entity MUST discard any knowledge obtained from the
initiating entity which was not obtained from the SASL
negotiation itself.
5. If the SASL negotiation involves negotiation of a security layer,
the initiating entity MUST discard any knowledge obtained from
the receiving entity which was not obtained from the SASL
negotiation itself.
6.1.2 Narrative
When an initiating entity authenticates with a receiving entity, the
steps involved are as follows:
1. The initiating entity requests SASL authentication by including a
'version' attribute in the opening XML stream header sent to the
receiving entity, with the value set to "1.0".
2. After sending an XML stream header in response, the receiving
entity sends a list of available SASL authentication mechanisms,
each of which is a element included as a child
within a container element that is sent as a child
of the first-level element. If channel encryption
must be established before a particular authentication mechanism
may be used, the receiving entity MUST NOT provide that mechanism
in the list of available SASL authentication methods. If the
initiating entity presents a valid initiating entity certificate
during TLS negotiation, the receiving entity MAY offer the SASL
EXTERNAL mechanism to the initiating entity during stream
authentication (see RFC 2222 [22]).
3. The initiating entity selects a mechanism by sending an
element to the receiving entity; this element MAY optionally
contain character data (in SASL terminology the "initial
response") if the mechanism supports or requires it. If the
initiating entity selects the EXTERNAL mechanism for
authentication, the authentication credentials shall be taken
from the certificate presented during TLS negotiation.
4. If necessary, the receiving entity challenges the initiating
entity by sending a element to the initiating
entity; this element MAY optionally contain character data.
5. The initiating entity responds to the challenge by sending a
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element to the receiving entity; this element MAY
optionally contain character data.
6. If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
This series of challenge/response pairs continues until one of three
things happens:
1. The initiating entity aborts the handshake by sending an
element to the receiving entity.
2. The receiving entity reports failure of the handshake by sending
a element to the initiating entity. The particular
cause of failure optionally may be communicated in the 'code'
attribute of the element, and may be 432 (password
transition is needed), 534 (authentication mechanism is too
weak), or 454 (temporary authentication failure).
3. The receiving entity reports success of the handshake by sending
a element to the initiating entity; this element MAY
optionally contain character data (in SASL terminology
"additional data with success").
Any character data contained within these elements MUST be encoded
using base64.
6.1.3 SASL Definition
Section 4 of the SASL specification [22] requires that the following
information be supplied by a protocol definition:
service name: "xmpp"
initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication
methods. The initiating entity chooses one method from the list
and sends it to the receiving entity as the value of the
'mechanism' attribute possesed by an element, optionally
including an initial response to avoid a round trip.
exchange sequence: Challenges and responses are carried through the
exchange of elements from receiving entity to
initiating entity and elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a element and success by sending a
element; the initiating entity aborts the exchange by sending an
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element.
security layer negotiation: If a security layer is negotiated, both
sides consider the original stream closed and new
headers are sent by both entities. The security layer takes
effect immediately following the ">" character of the empty
element for the client and immediately following the
closing ">" character of the element for the server.
use of the authorization identity: The authorization identity, if
present, is unused by xmpp.
6.1.4 Client-to-Server Example
The following example shows the data flow for a client authenticating
with a server using SASL.
Step 1: Client initiates stream to server:
Step 2: Server responds with a stream tag sent to the client:
Step 3: Server informs client of available authentication mechanisms:
DIGEST-MD5PLAIN
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Step 4: Client selects an authentication mechanism ("initial
response"):
Step 5: Server sends a base64-encoded challenge to the client:
cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
The decoded challenge is:
realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\ qop="auth",charset=utf-
8,algorithm=md5-sess
Step 6: Client responds to the challenge:
dXNlcm5hbWU9InJvYiIscmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik
9BNk1HOXRFUUdtMmhoIixcIGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5j
PTAwMDAwMDAxLHFvcD1hdXRoLFwgZGlnZXN0LXVyaT0ieG1wcC9jYXRhY2
x5c20uY3giLFwgcmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZ
jIxNDNhZjcsY2hhcnNldD11dGYtOA==
The decoded response is:
username="rob",realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\
cnonce="OA6MHXh6VqTrRk",nc=00000001,qop=auth,\ digest-uri="xmpp/
cataclysm.cx",\
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
Step 7: Server sends another challenge to the client:
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
The decoded challenge is:
rspauth=ea40f60335c427b5527b84dbabcdfffd
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Step 8: Client responds to the challenge:
Step 9: Server informs client of successful authentication:
Step 9 (alt): Server informs client of failed authentication:
Step 10: Client initiates a new stream to the server:
Step 11: Server responds by sending a stream header to the client,
with the stream already authenticated (not followed by further stream
features):
6.1.5 Server-to-Server Example
By bilateral agreement, server administrators MAY choose to use SASL
between two domains for the purpose of securing server-to-server
communications.
The following example shows the data flow for a server authenticating
with another server using SASL.
Step 1: Server1 initiates stream to Server2:
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Step 2: Server2 responds with a stream tag sent to Server1:
Step 3: Server2 informs Server1 of available authentication
mechanisms:
DIGEST-MD5KERBEROS_V4
Step 4: Server1 selects an authentication mechanism ("initial
response"):
Step 5: Server2 sends a base64-encoded challenge to Server1:
cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIi
xxb3A9ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz
The decoded challenge is:
realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\ qop="auth",charset=utf-
8,algorithm=md5-sess
Step 6: Server1 responds to the challenge:
cmVhbG09ImNhdGFjbHlzbS5jeCIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixjbm9uY2U9Ik9B
Nk1IWGg2VnFUclJrIixuYz0wMDAwMDAwMSxxb3A9YXV0aCxkaWdlc3QtdXJpPSJ4bXBwL2Nh
dGFjbHlzbS5jeCIscmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcs
Y2hhcnNldD11dGYtOAo=
The decoded response is:
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realm="cataclysm.cx",nonce="OA6MG9tEQGm2hh",\
cnonce="OA6MHXh6VqTrRk",nc=00000001,qop=auth,\ digest-uri="xmpp/
cataclysm.cx",\
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
Step 7: Server2 sends another challenge to Server1:
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
The decoded challenge is:
rspauth=ea40f60335c427b5527b84dbabcdfffd
Step 8: Server1 responds to the challenge:
Step 9: Server2 informs Server1 of successful authentication:
Step 9 (alt): Server2 informs Server1 of failed authentication:
Step 10: Server1 initiates a new stream to Server2:
Step 11: Server2 responds by sending a stream header to Server1, with
the stream already authenticated (not followed by further stream
features):
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6.2 Dialback Authentication
XMPP includes a protocol-level method for verifying that a connection
between two servers can be trusted (at least as much as the DNS can
be trusted). The method is called dialback and is used only within
XML streams that are declared under the "jabber:server" namespace.
The purpose of the dialback protocol is to make server spoofing more
difficult, and thus to make it more difficult to forge XML stanzas.
Dialback is not intended as a mechanism for securing or encrypting
the streams between servers as is done via SASL and TLS, only for
helping to prevent the spoofing of a server and the sending of false
data from it. Domains requiring more robust security SHOULD use TLS
or SASL as defined above.
Server dialback is made possible by the existence of DNS, since one
server can verify that another server which is connecting to it is
authorized to represent a given server on the Jabber network. All
DNS hostname resolutions MUST first resolve the hostname using an SRV
[24] record of _jabber._tcp.server. If the SRV lookup fails, the
fallback is a normal A lookup to determine the IP address, using the
jabber-server port of 5269 assigned by the Internet Assigned Numbers
Authority [7].
Note: the method for generating and verifying the keys used in the
dialback protocol MUST take into account the hostnames being used,
along with a secret known only by the receiving server and the random
ID generated for the stream. Generating unique but verifiable keys
is important to prevent common man-in-the-middle attacks and server
spoofing.
In the description that follows we use the following terminology:
o Originating Server -- the server that is attempting to establish a
connection between the two servers
o Receiving Server -- the server that is trying to authenticate that
Originating Server represents the Jabber server which it claims to
be
o Authoritative Server -- the server that is given when a DNS lookup
is performed on the name that Originating Server initially gave;
for basic environments this will be Originating Server, but it
could be a separate machine in Originating Server's network
The following is a brief summary of the order of events in dialback:
1. Originating Server establishes a connection to Receiving Server.
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2. Originating Server sends a 'key' value over the connection to
Receiving Server.
3. Receiving Server establishes a connection to Authoritative
Server.
4. Receiving Server sends the same 'key' value to Authoritative
Server.
5. Authoritative Server replies that key is valid or invalid.
6. Receiving Server tells Originating Server whether it is
authenticated or not.
We can represent this flow of events graphically as follows:
Originating Receiving
Server Server
----------- ---------
| |
| establish connection |
| ----------------------> |
| |
| send stream header |
| ----------------------> |
| |
| establish connection |
| | -------------
| | |
| establish connection |
| ----------------------> |
| |
| send stream header |
| ----------------------> |
| |
| establish connection |
|
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| ----------------------> |
| |
| validate dialback key |
|
Note: the value of the xmlns:db namespace declaration indicates
to Receiving Server that Originating Server supports dialback.
3. Receiving Server sends a stream header back to Originating
Server (the 'to' and 'from' attributes are NOT REQUIRED on the
root stream element):
4. Originating Server sends a dialback key to Receiving Server:
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98AF014EDC0...
Note: this key is not examined by Receiving Server, since
Receiving Server does not keep information about Originating
Server between sessions.
5. Receiving Server now establishes a connection back to
Originating Server, getting Authoritative Server.
6. Receiving Server sends Authoritative Server a stream header (the
'to' and 'from' attributes are NOT REQUIRED on the root stream
element):
7. Authoritative Server sends Receiving Server a stream header:
8. Receiving Server sends Authoritative Server a stanza indicating
it wants Authoritative Server to verify a key:
98AF014EDC0...
Note: passed here are the hostnames, the original identifier
from Receiving Server's stream header to Originating Server in
step 2, and the key Originating Server gave Receiving Server in
step 3. Based on this information and shared secret information
within the 'Originating Server' network, the key is verified.
Any verifiable method can be used to generate the key.
9. Authoritative Server sends a stanza back to Receiving Server
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verifying whether the key was valid or invalid:
or
10. Receiving Server informs Originating Server of the result:
Note: At this point the connection has either been validated via
a type='valid', or reported as invalid. Once the connection is
validated, data can be sent by Originating Server and read by
Receiving Server; before that, all data stanzas sent to
Receiving Server SHOULD be dropped. As a final guard against
domain spoofing, Receiving Server MUST verify that all XML
stanzas received from Originating Server include a 'from'
attribute and that the value of that attribute includes the
validated domain. In addition, all XML stanzas MUST include a
'to' attribute.
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7. XML Stanzas
7.1 Overview
Once the XML streams in each direction have been authenticated and
(if desired) encrypted, XML stanzas can be sent over the streams.
Three XML stanza types are defined for the 'jabber:client' and
'jabber:server' namespaces: , , and .
In essence, the stanza type can be seen as a "push"
mechanism whereby one entity pushes information to another entity,
similar to the communications that occur in a system such as email.
The element can be seen as a basic broadcast or "publish-
subscribe" mechanism, whereby multiple entities receive information
(in this case, presence information) about an entity to which they
have subscribed. The element can be seen as a "request-
response" mechanism similar to HTTP, whereby two entities can engage
in a structured conversation using 'get' or 'set' requests and
'result' or 'error' responses.
The syntax for these stanza types is defined below.
7.2 Common Attributes
Five attributes are common to message, presence, and IQ stanzas.
These are defined below.
7.2.1 to
The 'to' attribute specifies the JID of the intended recipient for
the stanza. In the 'jabber:client' namespace, a stanza SHOULD
possess a 'to' attribute, although a stanza sent from a client to a
server for handling by that server (e.g., presence sent to the server
for broadcasting to other entities) MAY legitimately lack a 'to'
attribute. In the 'jabber:server' namespace, a stanza MUST possess a
'to' attribute.
7.2.2 from
The 'from' attribute specifies the JID of the sender.
In the 'jabber:client' namespace, a client MUST NOT include a 'from'
attribute on the stanzas it sends to a server; if a server receives a
stanza from a client and the stanza possesses a 'from' attribute, it
MUST ignore the value of the 'from' attribute and MAY return an error
to the sender. In addition, a server MUST stamp stanzas received
from a client with the user@domain/resource (full JID) of the
connected resource that generated the stanza.
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In the 'jabber:server' namespace, a stanza MUST possess a 'from'
attribute. In particular, a server MUST include a 'from' attribute
on stanzas it routes to other servers. The domain identifier of the
JID contained in the 'from' attribute MUST match the hostname of the
server (or a subdomain thereof) as communicated in the SASL
negotiation or dialback negotiation.
7.2.3 id
The optional 'id' attribute MAY be used to track stanzas sent and
received. The 'id' attribute is generated by the sender. An 'id'
attribute included in an IQ request of type "get" or "set" SHOULD be
returned to the sender in any IQ response of type "result" or "error"
generated by the recipient of the request. A recipient of a message
or presence stanza MAY return that 'id' in any replies, but is NOT
REQUIRED to do so.
The value of the 'id' attribute is not intended to be unique --
globally, within a domain, or within a stream. It is generated by a
sender only for internal tracking of information within the sending
application.
7.2.4 type
The 'type' attribute specifies detailed information about the purpose
or context of the message, presence, or IQ stanza. The particular
allowable values for the 'type' attribute vary depending on whether
the stanza is a message, presence, or IQ, and thus are specified in
the following sections.
7.2.5 xml:lang
Any message or presence stanza MAY possess an 'xml:lang' attribute
specifying the default language of any CDATA sections of the stanza
or its child elements. An IQ stanza SHOULD NOT possess an 'xml:lang'
attribute, since it is merely a vessel for data in other namespaces
and does not itself contain children that have CDATA. The value of
the 'xml:lang' attribute MUST be an NMTOKEN and MUST conform to the
format defined in RFC 3066 [23].
7.3 Message Stanzas
Message stanzas in the 'jabber:client' or 'jabber:server' namespace
are used to "push" information to another entity. Common uses in the
context of instant messaging include single messages, messages sent
in the context of a chat conversation, messages sent in the context
of a multi-user chat room, headlines, and errors. These messages
types are identified more fully below.
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7.3.1 Types of Message
The 'type' attribute of a message stanza is OPTIONAL; if included, it
specifies the conversational context of the message. The sending of
a message stanza without a 'type' attribute signals that the message
stanza is a single message. However, the 'type' attribute MAY also
have one of the following values:
o chat
o error
o groupchat
o headline
For information about the meaning of these message types, refer to
XMPP IM [3].
7.3.2 Children
As described under extended namespaces (Section 7.6), a message
stanza MAY contain any properly-namespaced child element as long as
the namespace name is not "jabber:client", "jabber:server", or
"http://etherx.jabber.org/streams", and as long as the element name
does not match that of one of the core data elements, stream
elements, or defined children thereof.
In accordance with the default namespace declaration, by default a
message stanza is in the 'jabber:client' or 'jabber:server'
namespace, which defines certain allowable children of message
stanzas. If the message stanza is of type "error", it MUST include
an child; for details, see Section 7.7. If the message
stanza has no 'type' attribute or has a 'type' attribute with a value
of "chat", "groupchat", or "headline", it MAY contain any of the
following child elements without an explicit namespace declaration:
7.3.2.1 Body
The element contains the textual contents of the message;
normally included but NOT REQUIRED. The element MUST NOT
possess any attributes, with the exception of the 'xml:lang'
attribute. Multiple instances of the element MAY be included
but only if each instance possesses an 'xml:lang' attribute with a
distinct language value. The element MUST NOT contain mixed
content.
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7.3.2.2 Subject
The element specifies the topic of the message. The
element MUST NOT possess any attributes, with the
exception of the 'xml:lang' attribute. Multiple instances of the
element MAY be included for the purpose of providing
alternate versions of the same subject, but only if each instance
possesses an 'xml:lang' attribute with a distinct language value.
The element MUST NOT contain mixed content.
7.3.2.3 Thread
The element contains a random string that is generated by
the sender and that SHOULD be copied back in replies; it is used for
tracking a conversation thread (sometimes referred to as an "IM
session") between two entities. If used, it MUST be unique to that
conversation thread within the stream and MUST be consistent
throughout that conversation. The use of the element is
optional and is not used to identify individual messages, only
conversations. Only one element MAY be included in a
message stanza, and it MUST NOT possess any attributes. The element MUST be treated as an opaque string by entities; no
semantic meaning may be derived from it, and only exact, case-
insensitve comparisons be made against it. The element MUST
NOT contain mixed content.
The method for generating thread IDs SHOULD be as follows:
1. concatenate the sender's full JID (user@host/resource) with the
recipient's full JID
2. concatenate these JID strings with a full ISO-8601 timestamp
including year, month, day, hours, minutes, seconds, and UTC
offset in the following format: yyyy-mm-dd-Thh:mm:ss-hh:mm
3. hash the resulting string according to the SHA1 algorithm
4. convert the hexidecimal SHA1 output to all lowercase
7.4 Presence Stanzas
Presence stanzas are used in the 'jabber:client' or 'jabber:server'
namespace to express an entity's current availability status (offline
or online, along with various sub-states of the latter and optional
user-defined descriptive text) and to communicate that status to
other entities. Presence stanzas are also used to negotiate and
manage subscriptions to the presence of other entities.
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7.4.1 Types of Presence
The 'type' attribute of a presence stanza is optional. A presence
stanza that does not possess a 'type' attribute is used to signal to
the server that the sender is online and available for communication.
If included, the 'type' attribute specifies a lack of availability, a
request to manage a subscription to another entity's presence, a
request for another entity's current presence, or an error related to
a previously-sent presence stanza. The 'type' attribute MAY have one
of the following values:
o unavailable -- Signals that the entity is no longer available for
communication.
o subscribe -- The sender wishes to subscribe to the recipient's
presence.
o subscribed -- The sender has allowed the recipient to receive
their presence.
o unsubscribe -- A notification that an entity is unsubscribing from
another entity's presence.
o unsubscribed -- The subscription request has been denied or a
previously-granted subscription has been cancelled.
o probe -- A request for an entity's current presence. In general
SHOULD NOT be sent by a client.
o error -- An error has occurred regarding processing or delivery of
a previously-sent presence stanza.
Information about the subscription model used within XMPP can be
found in XMPP IM [3].
7.4.2 Children
As described under extended namespaces (Section 7.6), a presence
stanza MAY contain any properly-namespaced child element as long as
the namespace name is not "jabber:client", "jabber:server", or
"http://etherx.jabber.org/streams", and as long as the element name
does not match that of one of the core data elements, stream
elements, or defined children thereof.
In accordance with the default namespace declaration, by default a
presence stanza is in the 'jabber:client' or 'jabber:server'
namespace, which defines certain allowable children of presence
stanzas. If the presence stanza is of type "error", it MUST include
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an child; for details, see Section 7.7. If the presence
stanza possesses no 'type' attribute, it MAY contain any of the
following child elements (note that the child MAY be sent
in a presence stanza of type "unavailable" or, for historical
reasons, "subscribe"):
7.4.2.1 Show
The element specifies describes the availability status of an
entity or specific resource. Only one element MAY be
included in a presence stanza, and it MUST NOT possess any
attributes. The value SHOULD be one of the following (values other
than these four MAY be ignored; additional availability types could
be defined through a properly-namespaced child element of the
presence stanza):
o away
o chat
o xa
o dnd
For information about the meaning of these values, refer to XMPP IM
[3].
7.4.2.2 Status
The optional element specifies a natural-language
description of availability status. It is normally used in
conjunction with the show element to provide a detailed description
of an availability state (e.g., "In a meeting"). The
element MUST NOT possess any attributes, with the exception of the
'xml:lang' attribute. Multiple instances of the element
MAY be included but only if each instance possesses an 'xml:lang'
attribute with a distinct language value.
7.4.2.3 Priority
The optional element specifies the priority level of the
connected resource. The value may be any integer between -128 to
127. Only one element MAY be included in a presence
stanza, and it MUST NOT possess any attributes. For information
regarding the use of priority values in stanza routing within IM
applications, see XMPP IM [3].
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7.5 IQ Stanzas
7.5.1 Overview
Info/Query, or IQ, is a request-response mechanism, similar in some
ways to HTTP [25]. IQ stanzas in the 'jabber:client' or
'jabber:server' namespace enable an entity to make a request of, and
receive a response from, another entity. The data content of the
request and response is defined by the namespace declaration of a
direct child element of the IQ element, and the interaction is
tracked by the requesting entity through use of the 'id' attribute,
which responding entities SHOULD return in any response.
Most IQ interactions follow a common pattern of structured data
exchange such as get/result or set/result (although an error may be
returned in response to a request if appropriate):
Requesting Responding
Entity Entity
---------- ----------
| |
| |
| ------------------------> |
| |
| |
| |
| ------------------------> |
| |
| |
|
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values MAY be ignored):
o get -- The stanza is a request for information.
o set -- The stanza provides required data, sets new values, or
replaces existing values.
o result -- The stanza is a response to a successful get or set
request.
o error -- An error has occurred regarding processing or delivery of
a previously-sent get or set.
7.5.3 Children
As described under extended namespaces (Section 7.6), an IQ stanza
MAY contain any properly-namespaced child element as long as the
namespace name is not "jabber:client", "jabber"server", or "http://
etherx.jabber.org/streams", and as long as the element name does not
match that of one of the core data elements, stream elements, or
defined children thereof. However, an IQ stanza contains no children
in the 'jabber:client' or 'jabber:server' namespace since it is a
vessel for XML in another namespace.
If the IQ stanza is of type "error", it MUST include an
child; for details, see Section 7.7.
7.6 Extended Namespaces
While the core data elements in the "jabber:client" or
"jabber:server" namespace (along with their attributes and child
elements) provide a basic level of functionality for messaging and
presence, XMPP uses XML namespaces to extend the core data elements
for the purpose of providing additional functionality. Thus a
message, presence, or IQ stanza MAY house one or more optional child
elements containing content that extends the meaning of the message
(e.g., an encrypted form of the message body). This child element
MAY be any element (other than the core data elements, stream
elements, or defined children thereof). The child element MUST
possess an 'xmlns' namespace declaration (other than the stream
namespace and the default namespace) that defines all data contained
within the child element.
Support for any given extended namespace is OPTIONAL on the part of
any implementation. If an entity does not understand such a
namespace, it MUST ignore the associated XML data (if the stanza is
being routed on to another entity, ignore means "pass it on
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untouched"). If an entity receives a message or presence stanza that
contains XML data in an extended namespace it does not understand,
the portion of the stanza that is in the unknown namespace SHOULD be
ignored. If an entity receives a message stanza without a
element but containing only a child element bound by a namespace it
does not understand, it MUST ignore the entire stanza. If an entity
receives an IQ stanza in a namespace it does not understand, the
entity SHOULD return an IQ stanza of type "error" with an error
condition of .
7.7 Stanza Errors
As defined below, stanza-related errors are handled in a manner
similar to stream errors (Section 4.6).
7.7.1 Rules
The following rules apply to stanza-related errors:
o A stanza of type "error" MUST contain an child element.
o The receiving or processing entity that returns an error to the
sending entity SHOULD include the original XML sent along with the
element and its children so that the sender can inspect
and if necessary correct the XML before re-sending.
o An entity that receives a message stanza of type 'error' MUST NOT
respond to the stanza by sending a further message stanza of type
'error'; this helps to prevent looping.
o An child MUST NOT be included if the stanza type is
something other than "error".
7.7.2 Syntax
The syntax for stanza-related errors is as follows:
[include sender XML here]
The stanza name is one of message, presence, or iq.
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The value of the 'class' attribute must be one of the following:
o access -- the condition relates to access rights, permissions, or
authorization
o address -- the condition relates to the JID or domain to which the
stanza was addressed
o app -- the condition is particular to an application and is
specified in a namespace other than 'urn:ietf:rfc:xmppcore-rfc-
number:stanzas'
o format -- the condition relates to XML format or structure
o recipient -- the condition relates to the state or capabilities of
the recipient (which may be the server)
o server -- the condition relates to the internal state of the
server
The element MUST contain a child element that
specifies a particular stanza-related error condition, as defined in
the next section. (Note: the XML namespace name
'urn:ietf:rfc:xmppcore-rfc-number:stanzas' that scopes the element adheres to the format defined in RFC 2648 [16].)
7.7.3 Conditions
The following stanza-related error conditions are defined:
o -- the sender has sent XML that is malformed or
cannot be processed (e.g., a client-generated stanza includes a
'from' address, or an IQ stanza includes an unrecognized value of
the 'type' attribute); the associated class is "format".
o -- the feature requested is not
implemented by the recipient or server and therefore cannot be
processed; the associated class is "recipient".
o -- the stanza is understood but the action is
forbidden; the associated class is "access".
o -- the server could not process the
stanza because of a misconfiguration or an otherwise-undefined
internal server error; the associated class is "server".
o -- the value of the 'to' attribute in the
sender's stanza does not adhere to the syntax defined in
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Addressing (Section 3); the associated class is "address".
o -- the value of the 'to' attribute in the
sender's stanza does not correspond to a Jabber ID on the user's
server or a remote server; the associated class is "address".
o -- the action is not permitted when attempted by
the sender; the associated class is "access".
o -- the specific recipient requested is
currently unavailable; the associated class is "recipient".
o -- the user is not authorized to access
the requested service because registration is required; the
associated class is "access".
o -- a remote server or service specified
as part or all of the JID of the intended recipient does not
exist; the associated class is "address".
o -- a remote server or service specified
as part or all of the JID of the intended recipient could not be
contacted within a reasonable amount of time; the associated class
is "server".
o -- the service requested is currently
unavailable on the server; the associated class is "server".
7.7.4 Extensibility
If desired, an XMPP application MAY provide custom error information;
this MUST be contained in a properly-namespaced child of the element (i.e., the namespace name MUST NOT be one of
namespace names defined herein).
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8. XML Usage within XMPP
8.1 Namespaces
XML Namespaces [15] are used within all XMPP-compliant XML to create
strict boundaries of data ownership. The basic function of
namespaces is to separate different vocabularies of XML elements that
are structurally mixed together. Ensuring that XMPP-compliant XML is
namespace-aware enables any XML to be structurally mixed with any
data element within XMPP.
Additionally, XMPP is more strict about namespace prefixes than the
XML namespace specification requires.
8.2 Validation
A server is not responsible for validating the XML elements forwarded
to a client or another server; an implementation MAY choose to
provide only validated data elements but is NOT REQUIRED to do so.
Clients SHOULD NOT rely on the ability to send data which does not
conform to the schemas, and SHOULD ignore any non-conformant elements
or attributes on the incoming XML stream. Validation of XML streams
and stanzas is NOT REQUIRED or recommended, and schemas are included
herein for descriptive purposes only.
8.3 Character Encodings
Software implementing XML streams MUST support the UTF-8 (RFC 2279
[26]) and UTF-16 (RFC 2781 [27]) transformations of Universal
Character Set (ISO/IEC 10646-1 [28]) characters. Software MUST NOT
attempt to use any other encoding for transmitted data. The
encodings of the transmitted and received streams are independent.
Software MAY select either UTF-8 or UTF-16 for the transmitted
stream, and SHOULD deduce the encoding of the received stream as
described in the XML specification [1]. For historical reasons,
existing implementations MAY support UTF-8 only.
8.4 Inclusion of Text Declaration
An application MAY send a text declaration. Applications MUST follow
the rules in the XML specification [1] regarding the circumstances
under which a text declaration is included.
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9. IANA Considerations
The IANA registers "xmpp" as a GSSAPI [30] service name, as specified
in Section 6.1.3.
Additionally, the IANA registers "jabber-client" and "jabber-server"
as keywords for TCP ports 5222 and 5269 respectively.
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10. Internationalization Considerations
Usage of the 'xml:lang' attribute is described above. If a client
includes an 'xml:lang' attribute in a stanza, the server MUST NOT
modify or delete it.
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11. Security Considerations
11.1 High Security
For the purposes of XMPP communications (client-to-server and server-
to-server), the term "high security" refers to the use of security
technologies that provide both mutual authentication and integrity-
checking; in particular, when using certificate-based authentication
to provide high security, a chain-of-trust must be established out-
of-band (i.e., no self-signed certificates).
Implementations MUST support high security. Service provisioning
SHOULD use high security, subject to local security policies.
11.2 Client-to-Server Communications
The TLS protocol for encrypting XML streams (defined under Section 5)
provides a reliable mechanism for helping to ensure the
confidentiality and data integrity of data exchanged between two
entities.
The SASL protocol for authenticating XML streams (defined under
Section 6.1) provides a reliable mechanism for validating that a
client connecting to a server is who it claims to be.
The IP address and method of access of clients MUST NOT be made
available by a server, nor are any connections other than the
original server connection required. This helps protect the client's
server from direct attack or identification by third parties.
End-to-end encryption of message bodies and presence status
information MAY be effected through use of the methods defined in
End-to-End Object Encryption in XMPP [29].
11.3 Server-to-Server Communications
A compliant implementation MUST support both TLS and SASL for inter-
domain communications. For historical reasons, a compliant
implementation SHOULD also support the lower-security Dialback
Protocol (Section 6.2), which provides a mechanism for helping to
prevent the spoofing of domains.
Because service provisioning is a matter of policy, it is OPTIONAL
for any given domain to communicate with other domains, and server-
to-server communications MAY be disabled by the administrator of any
given deployment. If a particular domain enables inter-domain
communications, it SHOULD enable high security. In the absence of
high security, a domain MAY use server dialback for inter-domain
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communications.
11.4 Firewalls
Communications using XMPP normally occur over TCP sockets on port
5222 (client-to-server) or port 5269 (server-to-server), as
registered with the IANA [7]. Use of these well-known ports allows
administrators to easily enable or disable XMPP activity through
existing and commonly-deployed firewalls.
11.5 Mandatory to Implement Technologies
At a minimum, all implementations MUST support the following
mechanisms:
for authentication: the SASL DIGEST-MD5 mechanism
for confidentiality: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA
cipher)
for both: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher
supporting client-side certificates)
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References
[1] World Wide Web Consortium, "Extensible Markup Language (XML)
1.0 (Second Edition)", W3C xml, October 2000, .
[2] Jabber Software Foundation, "Jabber Software Foundation",
August 2001, .
[3] Saint-Andre, P. and J. Miller, "XMPP Instant Messaging (draft-
ietf-xmpp-im-06, work in progress)", March 2003.
[4] Day, M., Aggarwal, S., Mohr, G. and J. Vincent, "A Model for
Presence and Instant Messaging", RFC 2779, February 2000,
.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] University of Southern California, "Transmission Control
Protocol", RFC 793, September 1981, .
[7] Internet Assigned Numbers Authority, "Internet Assigned Numbers
Authority", January 1998, .
[8] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998, .
[9] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet host
table specification", RFC 952, October 1985.
[10] Braden, R., "Requirements for Internet Hosts - Application and
Support", STD 3, RFC 1123, October 1989.
[11] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile
for Internationalized Domain Names (draft-ietf-idn-nameprep-11,
work in progress)", June 2002.
[12] Hoffman, P. and M. Blanchet, "Preparation of Internationalized
Strings ("stringprep")", RFC 3454, December 2002.
[13] Saint-Andre, P. and J. Hildebrand, "Nodeprep: A Stringprep
Profile for Node Identifiers in XMPP (draft-ietf-xmpp-nodeprep-
01, work in progress)", February 2003.
[14] Saint-Andre, P. and J. Hildebrand, "Resourceprep: A Stringprep
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Profile for Resource Identifiers in XMPP (draft-ietf-xmpp-
resourceprep-01, work in progress)", February 2003.
[15] World Wide Web Consortium, "Namespaces in XML", W3C xml-names,
January 1999, .
[16] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
August 1999.
[17] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999.
[18] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, December 1996.
[19] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
53, RFC 1939, May 1996.
[20] Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
[21] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595,
June 1999.
[22] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997.
[23] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
[24] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2052, October 1996.
[25] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[26] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[27] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646",
RFC 2781, February 2000.
[28] International Organization for Standardization, "Information
Technology - Universal Multiple-octet coded Character Set (UCS)
- Amendment 2: UCS Transformation Format 8 (UTF-8)", ISO
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Standard 10646-1 Addendum 2, October 1996.
[29] Saint-Andre, P. and J. Hildebrand, "End-to-End Object
Encryption in XMPP (draft-ietf-xmpp-e2e-00, work in progress)",
February 2003.
[30] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, January 1997.
Authors' Addresses
Peter Saint-Andre
Jabber Software Foundation
EMail: stpeter@jabber.org
URI: http://www.jabber.org/people/stpeter.php
Jeremie Miller
Jabber Software Foundation
EMail: jeremie@jabber.org
URI: http://www.jabber.org/people/jer.php
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Appendix A. XML Schemas
The following XML schemas are descriptive, not normative.
A.1 Streams namespace
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A.2 TLS namespace
A.3 SASL namespace
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A.4 Dialback namespace
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A.5 Client namespace
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A.6 Server namespace